Hydrothermal Synthesis of Nanomaterials Andrew van Bommel January 18th, 2006

Hydrothermal Synthesis The reactants are dissolved (or placed) in water or another solvent (solvothermal) in a closed vessel Bomb is heated above BP Conventional or MW oven Commercially: Tons of zeolites daily Nazari, G.A., Pistoria, G. Lithium Batteries; Klumer Academic Pubs.: Boston, 2004.

Conditions Solvent above boiling point (supercritical water) Usually basic conditions Useful for Nano! Oxides (TiO2, VOx, MxMnO2) Layered oxides: nanowires/tubes C-nanotubes can be formed this way Some elemental nanostructures (eg: Bi)

Mechanism for HT rxn Usually follows a liquid nucleation model Differs from solid-state- reaction mechanism from diffusion of atoms or ions between reactants Due to enhanced solubility: Solubility of water increases with temperature, but alkaline solubility increases dramatically with temperature Barrer, R.M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982

Solubility Solubility of SiO2 (quartz) T (°C) Sol (g/L) 100 - 150 0.0030 200 0.024 250 0.049 300 0.068 350 0.070 374 0.023 T (°C) NaOH wt% Sol (g/L) 300 1 5 15 20 135 514 350 25 152 515 400 35 155 560 450 163 Barrer, R.M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982

Next Slides Examples: Control of Shape Nanoparticles Nanowires/tubes Nanoflowers Control of Shape

BaTiO3 Nanoparticles Ba(OH)2 + TiO2  BaTiO3 nanoparticles 300 - 450°C, HT Two proposed mechanisms: Dissolution-recrystallization In situ crystallization Hakuta, R., Ura, H. Hayashi, H, and Arai, K. Ind. Eng. Chem. Res. 2005, 44, 840-846

Dissolution-Recrystallization BaTiO3 formation at 150°C Stated: “when varying the water/isopropanol ratio in synthesis at 150°C, the grain size of barium titanate decreases when the amount of alcohol increases, i.e. when the solubility of the precursors decreases” “TEM observations of incompletely reacted powders showed that the grains are either amorphous or entirely crystalline BaTiO3, which means that homogeneous nucleation and growth is occurring instead of heterogeneous nucleation “high resolution TEM observations of fully reacted powders revealed the presence of necks between particles. These three experimental observations in the same reaction system provide strong evidence of dissolution-precipitation as the primary reaction mechanism.” Pinceloup, R., Courtoisa, C., Vicensb, J., Lerichea, A., and Thierry, B. J. Eur. Ceram. Soc. Res. 1999, 19, 973-977.

In-situ crystallization Non-dissolved TiO2 reacts with barium and hydroxide ions Eckert, J.O., Hung-Houston, C.C., Gersten, B.L., Lencka, M.M., Riman, R.E., J. Am. Ceram. Soc. 1996, 79, 2939.

Why Nano? For in-situ transformation, formation of a porous product facilitates the transfer of the reacting species, leading to a facile reaction For dissolution-precipitation reaction, dissolution (and subsequent precipitation) must be fast to ensure a steady flow of reactants Eckert, J.O., Hung-Houston, C.C., Gersten, B.L., Lencka, M.M., Riman, R.E., J. Am. Ceram. Soc. 1996, 79, 2939.

Tube/wire Formation TiO2 +NaOHtitanate nanomaterial Crystal flakes rolled along [001] direction Chen, Q., Du, G.H., Zhang, S., Peng, L.M. Acta Cryst. 2002, B58, 587-593.

Why nanowires/tubes? Two-dimensional crystal flakes have low resistance to bending: hydrothermal energy curls these flakes Why tubes? When diameter grows, the strain in the tubes is outweighed by the minimizing of energy with nanotubes’ self-closed layers (no dangling bonds) Remskar, M. Adv. Mater. 2004, 16, 1497-1502.

More Nano! Nanoflower ZnO- CTAB assisted hydrothermal reaction of ZnO (in NaOH solution) Zhang, H., Yang, D., Ji, Y., Ma, X., Xu, J., Que, D. J. Phys. Chem. B. 2004, 108, 3955-3958

Hydrothermal Route Pros: Cons: New materials Easy, relatively cheap Difficult to control morphology, size Not for all materials May obtain variation in size

Summary Hydrothermal synthesis involves the chemical reaction of materials in aqueous solution heated (usually above BP) in a sealed vessel (bomb) Alkaline solution used to increase solubility (for a dissolution-precipitation reaction) It is difficult to predict the resultant morphology of the product

Summary Nanomaterials are apt to form in HT synthesis as the formation of these structures allows for facile diffusion of reactants Hydrothermal synthesis is used for oxide nanoparticle synthesis as the solubility is high in the alkaline medium